Is your dust collection system an explosion hazard?

Chemical Engineering Progress, Oct 2003 by Ebadat, Vahid

Follow these guidelines to assess the risk of a dust-induced fire or explosion in your equipment and learn how to avoid such an event.

STATISTICS SUGGEST THAT DUST collection systems (i.e. dust collectors, filter receivers, baghouses, etc.) have been involved in the second-highest percentage of dust explosions that occur across all industries, including pharmaceutical, metal, food, plastics, coal and wood (1). The prevalence of dust explosions in dust collection units may be due to the fact that the hazards posed by the powders handled are not fully understood or that explosion preventive and protective control measures are insufficient or absent. This article will help the reader assess the potential for dust ignition and explosions during dust collection operations.

Causes of ignition

Three main elements are required for a fire or an explosion to occur: a fuel (combustible powder); an oxidant (typically oxygen in air); and a sufficiently energetic ignition source. The fuel and oxidant, when present in an appropriate ratio, are referred to as a flammable atmosphere. Due to the inherent nature of dust-collection systems, it is prudent to assume that a flammable atmosphere is likely to occur frequently. The use of mechanical vibration systems or reverse jet/pulse-jet units to dislodge powder entrained in the filters will increase the frequency of generating flammable clouds of dust within the units. An additional hazard is that the powder within dust collectors is generally the finest seen in the process, which can result in a potential increase in the ignition sensitivity and explosion severity. If an ignition source were to be present simultaneously with the flammable atmosphere, a fire or an explosion could result (2).

In dust collection operations, several different ignition sources could be present:

* impact and friction sparks, which can result from foreign objects entering the conveying ductwork upstream of the dust collector

* accumulation of powder layers within ducts or in the dust collector itself, which can result in exothermic decomposition of self-heating substances over time, if the dust collector is serving an operation that involves heating of the powder, such as drying

* transfer of smoldering material from connected plant items during normal operations, such as milling, drying, etc.

* transfer of sparks or smoldering materials during abnormal operations, such as maintenance and welding operations

* sparks or hot surfaces from electrical and mechanical equipment in the dust-collection system, such as fans

* electrostatic charge generation, which can lead to incendive discharges (e.g., filter cage assemblies isolated from ground).

Since electrostatic ignition sources are generally less understood than other ignition sources, further information is provided on electrostatic-charge generation, accumulation and discharges, together with control measures (3).

Charge generation - Although the magnitude and polarity of charge is usually difficult to predict, charge generation should almost always be expected whenever powder particles come into contact with other surfaces. Electrostatic charge generation occurs, for example, upstream of the dust collector during operations, such as mixing, grinding, sieving, pouring and pneumatic transfer. The chemical composition and the condition of the contacting surfaces can often influence the charging characteristics.

Charge accumulation - Generally, powders are divided into three groups depending on their ability to retain static charge, even if the powder is in contact with an electrically grounded conductive object. This ability is known as volume resistivity or [rho]^sub v^: Powders with volume resistivities up to about 10^sup 6^ ohmm are considered conductive. Powders with volume resistivities in the range of 10^sup 6^-10^sup 9^ ohmm have medium resistivity. Powders with volume resistivities above 10^sup 9^ ohmm are high-resistivity powders. Charge will accumulate on a powder and on any plant item that may either be insulating or electrically isolated from ground if the charge generation rate exceeds the rate at which the charge dissipates to ground.

Electrostatic discharges - Accumulation of electrostatic charge on powder or equipment creates a dust explosion hazard, only if the accumulated charge is suddenly released in the form of a discharge with sufficient energy to ignite the dust cloud. Potentially incendive discharges resulting from charged powder and equipment include spark discharges, brush discharges, propagating brush discharges and cone discharges (also referred to as bulking discharges) (CEP, July 2003, p. 48).

Control measures

General precautions against electrostatic discharge hazards include the following:

Bonding and grounding - Spark discharges can be avoided by electrically grounding all conductive items, such as the dust collector, the metal ductwork, metal cages that support the filter socks, and the downstream equipment and vessels.

Avoiding the use of insulating materials - Where high-surface charging processes exist, non-conductive materials should not be used, unless the breakdown voltage across the material is less than 4 kV. This is because insulating materials with a breakdown voltage of greater than 4 kV, when sufficiently charged, could give rise to discharges with enough energy to ignite most flammable dust cloud atmospheres. Examples of nonconductive objects include plastic pipes, hoses, containers, filter bags and coatings.